This invention relates to a method of making a glass article by fusing a rod and tube such that substantially no seeds are formed at the interface between them. The method of this invention is useful for making low loss optical fibers, especially those fibers in which the core includes an annular region of depressed refractive index relative to silica.
Optical fibers having refractive index profiles such as W-profiles, segmented core profiles, and the like possess desirable dispersion characteristics. See U.S. Pat. Nos. 4,715,679 and 5,031,131 for teachings of various kinds of dispersion modified optical fibers. Fibers having these kinds of refractive index profiles have often been made by chemical vapor deposition (CVD) processes such as plasma CVD processes that are capable of forming single-mode fibers the cores of which include layers of different refractive indices (see FIGS. 7 and 8, for example). Such processes produce relatively small preforms. It is advantageous to form dispersion modified optical fiber preforms by outside vapor deposition (OVD) processes which produce relatively large preforms or draw blanks in order to decrease the cost of making the fiber.
A typical OVD process for forming such fibers is disclosed in U.S. Pat. No. 4,629,485. In accordance with that patent, a germania-doped silica rod is formed and stretched to decrease its diameter. A piece of the rod is used as a mandrel upon which pure silica glass particles or soot is deposited. The resultant composite structure is heated in a consolidation (drying and sintering) furnace through which a fluorine-containing gas flows. The soot is therefore doped with fluorine and sinters on the rod. One or more additional layers of glass are formed on the outer surface of the fluorine-doped silica layer to form a blank from which a fiber can be drawn.
When soot is sintered in accordance with the aforementioned method, whereby fluorine is supplied to the porous preform solely by way of the fluorine-containing muffle gas, the fluorine concentration (as measured by the xcex94 of the fluorine-containing layer) is not sufficient to provide certain desirable optical characteristics. The typical fluorine concentration acheived with muffle gas doping provides a xe2x88x920.4%xcex94 when SiF4 is the fluorine-containing constituent. The maximum delta value for SiF4 produced by the above-described process is xe2x88x920.5%xcex94.
One aspect of the invention concerns a method of making an optical fiber preform an annular region of which consists of silica doped with a sufficient amount of fluorine that the delta value of the annular region with respect to silica is more negative than xe2x88x920.5%xcex94.
As used herein, the term xcex94a-b, the relative refractive index difference between two materials with refractive indices na and nb, is defined as
xcex94a-b=(na2xe2x88x92nb2)/(2na2)xe2x80x83xe2x80x83(1)
For simplicity of expression, xcex94 is often expressed in percent, i.e. one hundred times xcex94. In this discussion, na is the refractive index of the fluorine-doped glass and nb is the refractive index of silica.
Another aspect of the invention concerns the collapse of a tube of fluorine-doped and/or boron-doped glass onto a rod of core glass such that during the resultant fusion of the interface between those two members, substantially no seeds are formed.
When a fluorine-doped silica tube is collapsed onto a germania-doped silica rod, the resultant interface between those two members has heretofore contained many seeds, and much of the resultant preform or blank produces unusable optical fiber. Such seed formation is less prevalent when members formed of other glass compositions such as a germania-doped silica rod and a pure silica tube are fused to form a preform.
U.S. Pat. No. 4,668,263 discloses a method for collapsing a silica tube having a fluorine-doped inner layer onto the surface of a silica rod. In accordance with that patent the collapse step is accomplished by rotating the tube and heating it with the flame from a longitudinally travelling burner. That technique could not be employed to make dispersion modified fiber designs of the type that utilize the entire fluorine-doped tube, including the outer surface, as part of the core region or light propagating region of the fiber. The reason for this is that, since the flame wets the glass, i.e. introduces hydroxyl contamination, the resultant fiber would be rendered unsuitable for operation at wavelengths where attenuation due to hydroxyl ions is large. A further disadvantage of this method concerns the temperature of the flame, which is not lower than 1900xc2x0 C. At such high temperatures, control of the process becomes difficult. The axis of the preform can become non-linear or bowed. If the core rod is a soft glass such as a germania-doped glass, the rod can become softer than the tube; this can result in an out-of-round core or a core that is not concentric with the outer surface of the resultant fiber.
U.S. Pat. No. 4,846,867 discloses a method for collapsing a fluorine-doped silica tube onto the surface of a silica rod. Prior to the tube collapse step, a gas phase etchant is flowed through the gap between the rod and tube while the tube is heated by a flame. In the specific examples, wherein SF4 is the etchant, a gaseous mixture of SF4, Cl2 and oxygen (ratio 1:1:6 by volume) is introduced through a gap between the rod and the tube. Such a gaseous mixture removes glass from the treated surfaces of the rod and tube, thus forming new surfaces at the rod/tube interface. The chlorine is present in an amount sufficient to remove water generated by the fluorine-containing etchant. The outer surface of the resultant preform is thereafter coated with silica soot particles that are dried, doped with fluorine and then sintered to form a blank from which an optical fiber is drawn. The flame that was directed onto the tube during the gas phase etching step introduces water into the outer surface of the tube. The attenuation of the fiber resulting from that water is high. The attenuation at 1380 nm for one example is 30 dB/km which is attributed to contact of the oxyhydrogen flame with the preform.
An object of the invention is to provide a method of joining first and second adjacent layers of a glass preform such that the interface therebetween is substantially seed-free. A further object is to provide an improved method of joining a core region to an adjacent region in a glass preform. Another object is to provide a method of making a rod-in-tube preform by the step of cleaning the adjacent surfaces of the rod and tube in such a manner that the outer surface of the tube does not become contaminated with water. Yet another object is to provide a method of forming a seed-free interface between a rod and tube in an optical fiber preform without removing glass from the adjacent surfaces of the rod and tube. Yet another object is to provide a method of making fluorine-doped silica glass having high negative delta by the OVD technique.
The present invention relates to a method of making a glass article. The method comprises inserting a non-porous glass core rod into a non-porous glass tube to form an assembly that is inserted into a furnace. While the entire assembly is being heated, a centerline chlorine-containing gas is flowed into the first end of the tube and between the tube and the rod, and out of the second end of the tube. Thereafter, the tube is collapsed onto the rod to form an assembly which can be formed into the glass article such as an optical fiber. The tube collapse step can be performed in the same furnace in which the chlorine cleaning step occurs.
As the adjacent surfaces of the rod and tube are cleaned by the centerline gas while the assembly is in a furnace, the outer surface of the tube is not contaminated by water that would be present if a flame were employed for heating the assembly during the cleaning step.
This method is especially suitable for forming an optical fiber having a core that includes an annular region of depressed refractive index.
The tube can be formed of silica doped with fluorine or boron, both of which can be added to silica to lower its refractive index. Fluorine is the preferred dopant since attenuation due to B2O3 limits fiber usage to wavelengths less than about 1200 nm.
To provide a tube doped with fluorine, a fluorine-containing gas is flowed into the aperture and outwardly through the pores of a porous, cylindrically-shaped glass preform. The porous glass preform is heated to sinter it into a non-porous fluorine-doped tube.
A further aspect of the invention concerns a method of making a glass article having an annular region containing a high content of fluorine. A tubular porous glass preform is initially formed. The preform is heated, and a centerline gas is flowed into the longitudinal aperture of the preform and outwardly through its pores. The centerline gas consists entirely of a fluorine-containing compound, whereby a high concentration of fluorine becomes incorporated in the pores of the preform. The porous preform is heated to sinter it into a non-porous fluorine-containing glass tube. A cylindrically-shaped core rod is inserted into the fluorine-doped tube. The tube is then shrunk onto the core rod, and the interface between the core preform and the tube is fused. An article such as an optical fiber can be formed from the resultant preform.